Development: this develops up the silver salts that have been effected during the exposure. It speeds up the darkening of the silver by the by 1000 million times. This is one reason to make sure the exposure in the camera is correct, if they have not received enough light there will be no reaction, if there is too much there will be an over reaction.

While the image at this point is formed as metallic silver the unexposed silver salts still remain, and they need to be removed to stabilize the image.

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There are three factors that effect the development of the film.

Time
Temperature
Dilution of the chemical